I personally stutter quite severely and normal talking can get VERY exhausting, so for me i just get relieved whenever friends help me finish sentences. It would be different if it was a person i met 5 minutes ago and it seems like they are finishing words just because they are impatient with me, but its quite easy to tell if someone is genuinely trying to help. But everyone is different, i know lots of people who stutters that hates it, so asking him is my best advice! Can't hurt :)